[Elecraft] More on S-meter and RF gain

[Ian White GM3SEK]

Pete Smith wrote:
>Experimenting with the RF gain control, and with the RX equalizer.  I
>have discovered that the setting of the equalizer to emphasize CW beat
>notes and de-emphasize the highs makes a tremendous difference in the
>perceived noise level and strength of the audio on CW.  One thing that
>puzzles me, though - when I turn down the RF gain, the S meter reading
>on a signal being received increases substantially.  For example, I just
>worked E74Y, whose very good signal was only S8 with the RF gain all the
>way up, but when I turned it back to roughly 1 o'clock, the peaks were
>S9 +10.  Does this make sense?
>

Yes... in a sense :-)

The behavior of the K3's S-meter in response to the "RF Gain" control is 
emulating traditional hardware receivers. What you now see in the K3 is 
essentially the same as you've been seeing for many years in the large 
majority of traditional receivers.

In traditional hardware receivers, the S-meter is actually reading the 
voltage on the AGC line. Stronger signals produce more AGC voltage, 
which deflects the meter more while simultaneously reducing the RF/IF 
gain to keep the audio signal level fairly constant.

The manual "RF Gain" control functions by applying a permanent 
negative-going DC voltage to the AGC line. This has the same effect as a 
signal-derived AGC voltage - it reduces the RF/IF gain and causes the 
S-meter reading to rise. The only difference is that the S-meter reading 
rises to a steady value, and will not fall back when signals go away. If 
a signal is strong enough to generate *more* AGC voltage than you have 
already applied through the "RF Gain" control, then the S-meter will 
rise - but it will never fall back below the baseline level that you 
have set.

In other words, that steady baseline meter reading does *not* represent 
the strength of any weak signals. It's better to think of that steady 
reading as the "AGC threshold", below which the S-meter readings are 
simply not available. True signal strength readings are only available 
for signals that are strong enough to deflect the meter *above* that 
threshold.

The vast majority of traditional receivers behave like this, and the K3 
emulates that behavior quite faithfully.

The only major difference is that the K3 doesn't have any 
gain-controlled RF stages. Both the manual "RF Gain" and the AGC are 
implemented in DSP at the 15kHz IF, with backup from the hardware AGC 
loop in the 8.215kHz IF stage. The only gain controls that genuinely 
change the levels at the RF signal frequency are the ATT and PREAMP 
buttons.

Bottom line: don't ever expect an "RF Gain" control to do literally what 
the label says. You never could... and you still can't.



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek